The ability to multitask while waking starts declining by age 55, a recent study found.

Meaning middle age adults may have a harder time walking and talking, among other activities, at the same time a full decade before the traditional old age threshold of 65, according to the study, published in Lancet Healthy Longevity.

The decline is caused by changes in brain function, not changes in physical condition, according to the study.

Difficulties walking and talking at the same time among some participants of the study could be a sign of accelerated brain aging, lead researcher Junhong Zhou said in a press release. "Dual-tasking" difficulties can also cause unsteadiness for those older than 65.

"As compared to walking quietly, walking under dual-task conditions adds stress to the motor control system because the two tasks must compete for shared resources in the brain," Zhou said. "What we believe is that the ability to handle this stress and adequately maintain performance in both tasks is a critical brain function that tends to be diminished in older age."

Other activities that become harder to do while walking included reading signs and making decisions.

Nearly 1,000 adults in Spain were studied, including 640 who completed gait and cognitive assessments, researchers said Tuesday. Each participant was able to walk without assistance. The adults, ages 40-64, were relatively stable when they walked under normal, quiet conditions.

"However, even in this relatively healthy cohort, when we asked participants to walk and perform a mental arithmetic task at the same time, we were able to observe subtle yet important changes in gait starting in the middle of the sixth decade of life," Zhou said.

The ability to do two tasks at once depends on a number of cognitive resources, researchers explained in the study. Performance can be diminished based on the speed the brain processes information.

"Evidence suggests that aging alters each of these factors and thus leads to greater DTC [dual-task cost] to performance in one or both involved tasks," according to the study.

The study also noted that "poor dual-task gait performance has been linked to risk of major cognitive impairment, falls, and brain health alterations in older adults," adding that the resutlsts "further suggest that dual-task walking is an important functional ability that should be routinely monitored starting in middle age."

Many age-related diseases, including neurodegenerative diseases such as dementia, first manifest during middle age, but it's not all downhill after 55. A number of the participants over the age of 60 performed the tests as well as participants 50 and younger.

Some individuals seem to be more resistant to aging, Zhou said.

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